The present invention relates generally to aboveground storage tanks, and more specifically to a method of constructing a tank. Unlike conventional methods, the present method does not require the use of scaffolds to provide either: (1) access to the shell plates for construction personnel; or (2) resistance to buckling damage from ambient wind during shell construction.
Aboveground storage tanks typically consist of a circular, essentially flat bottom and a vertical cylindrical shell having a lower edge that is joined to the tank bottom. The shell of a conventional storage tank consists of a stack of rings that are joined together at girth seams. Each shell ring is constructed of shell plates that are joined together at vertical seams. Tanks typically have a fixed roof that may be cone-shaped or dome-shaped and is joined to the top of the shell, or a floating roof that floats on the product stored in the tank.
During construction of the shell, it is conventional to use scaffold brackets to attach a scaffold to the outside or inside surface of the shell. The scaffold provides construction personnel with access to the shell plates during their placement in the shell rings and for fit-up and welding of vertical seams and girth seams between plates. Conventionally, a scaffold is initially mounted on the first shell ring and is consecutively "jumped" upwards as work progresses to higher shell rings.
The use of scaffolds for constructing a tank shell has a number of disadvantages. The scaffold consists of many components that must be fabricated, maintained in working order, stored in a construction equipment warehouse, shipped to the tank construction site, installed on the shell rings, moved to higher shell rings during construction of the higher shell rings, removed from the tank after tank construction, and sent back to a construction equipment warehouse for repair, maintenance, and storage until the next tank construction project. Time and effort is also required to remove the scaffold bracket straps after use, and to grind smooth any remaining weld burrs on the shell plates. The time required to successively jump a scaffold to higher shell rings alone adds significantly to the time needed to construct a tank shell. It is thus desirable to find an alternative tank construction method that does not require the use of a scaffold.
One consideration has weighed in favor of continuing the use of scaffolding. As wind flows over a cylindrical tank shell, it produces an air pressure on the upwind surface of the tank shell that is higher than the local barometric pressure at the tank site. It also produces an air pressure on the downwind surface of that same tank shell that is lower that the local barometric pressure. This differential of air pressures tends to cause the shell to deflect inwardly on the upwind side of the tank. While a tank is being constructed, the shell may lack adequate rigidity to prevent such wind-produced air pressures from causing the shell to buckle. A scaffold that completely encircles the shell during construction can, if properly designed and installed, provide the shell with resistance to such buckling. This is described, for example, in Vaughn, et al., U.S. Pat. No. 3,908,793.